1. Field of the Invention
The present invention relates to a method of manufacturing, in a semiconductor device manufacturing process which is not capable of preventing a carbon contamination such as a process which is conducted within a clean room using a hopper filter, a semiconductor device providing a clean semiconductor interface with the removal of that contamination. In particular, the present invention relates to a method of manufacturing a thin-film semiconductor device providing a clean semiconductor interface and a gate insulating film low in carbon density by removing a carbon contaminator on the interface between an active layer and the gate insulating film, as well as by removing carbon impurities in the gate insulating film which is formed using an organic silane source in a method of manufacturing a thin-film transistor (TFT) using a thin-film semiconductor such as a liquid-crystal display field.
2. Description of the Related Art
Up to now, in the method of manufacturing the semiconductor device, various contamination removing manners have been established for the removal of the contaminator on its surface as well as for the prevention of contamination as a problem. For the removal of heavy metal, there have been remarkably widely known methods in which heavy metal is removed by adding hydrochloric acid to hydrogen peroxide solution, and so on. Also, for the removal of a physical absorber, there have been well used a cleaning method using the cavitations of supersonic waves, a cleaning method using a brush, and so on. Moreover, even in the liquid-crystal display field where a large number of thin-film transistors are formed on an insulating substrate, using normal tetraethyl silicate chemical formula Si(OC2H5)4 (so-called TEOS) as a source gas, a so-called stepping of a thin-film transistor wiring and so on are reduced utilizing the excellence of the step coating property of the gas. Further, in the liquid-crystal display field using a process which is not at a high temperature used in a silicon wafer process but at 600xc2x0 C. or lower, TEOS has been used for axe2x80xa2gate insulating film or an under layer in addition to an inter-layer insulating film. In the thin-film transistor (so-called TFT) which is applied to the liquid-crystal display or the like, an under film formed on an insulating substrate such as a glass substrate, a gate insulating film, an interlayer insulating film and so on are formed through the heat CVD method, the plasma CVD method or the like with normal tetraethyl silicate as a source gas. However, this suffers from a problem on the oxide film characteristic because a large amount of carbon remains.
For the removal of organic substances such as carbon that adheres to the surface, there have been well known a cleaning method using solvent where sulfuric acid is added to hydrogen peroxide solution, a dry ashing method using oxygen plasma, and so on. However, from the research of the inventors, it has been found that the removal of carbon is under more complicated circumstances. The cause of the mixture of carbon contaminations is that a photo-resist used for forming an intended pattern in a photolithography process is of a photo sensitive organic that causes a carbon contamination. Also, in the method of manufacturing the semiconductor device, the thin-film process is now essential, and a vacuum device for the thin-film process is also essential. However, there exists a vacuum pump in the vacuum device for making vacuum, yet using oil. This causes carbon contamination. In addition, the contamination is caused by a vapor pressure generated from teflon (PFA), polypropylene (PP), polyvinylidene fluoride (PVDF), ethylene trifluoride covalent resin (ECTFE), ethylene tetrafluoride covalent resin (ETFE), polyethylene (PE) tetrafluoride, or the like, which is used as a substrate carrier, a floor material within a clear room, a wall material, a filter, and so on.
A conventional method is that a dry ashing is conducted after a photolithography process, and a solvent in which sulfuric acid is added to hydrogen peroxide solution at a rate of 1:1 is heated at 80xc2x0 C. immediately before the respective processes, and then used, to thereby remove an organic substance, (hereinafter referred to as xe2x80x9cwet ashingxe2x80x9d). Immediately after then, a succeeding process is conducted. Conventionally, it has been thought that all of the organic substances can be removed by the dry ashing and the wet ashing. However, it has been found as a result of estimating the carbon contamination on the substrate surface through the known XPS measurement, that only Cxe2x80x94C bonds (single bond of carbon) are hardly removed.
FIG. 2 shows a substrate surface 21 (indicated by a dotted line graph in FIG. 2) which has been subjected to a photo-resist coating, pre-baking, light-exposure, development, post-baking, and photo-resist peeling, and a surface 22 of the substrate (indicated by a solid line graph in FIG. 2) which has been further subjected to dry ashing and wet ashing, which have been measured using XPS. Under the measuring condition where the angle of a detector is set to 15xc2x0 in order to obtain the information of the surface as much as possible, an area 1 mm"PHgr" on the substrate surface was measured. A horizontal axis represents a bonding energy with an unit of eV whereas a vertical axis represents the intensity of the detector with an arbitrary unit.
It is found from the graph of FIG. 2 that a peak in the vicinity of 284.8 eV is increased in both the cases of before (dotted line) and after (solid line) the substrate surface is subjected to dry ashing and wet ashing, and all of other peaks are decreased. The peak of 284.8 eV shows the existence of Cxe2x80x94C single bond.
This shows that the removal of single bond of carbon is very difficult in the conventional dry ashing and wet ashing and almost impossible. Because carbon remains on the substrate surface as impurities, if, for example, an oxide film or the like is formed on the substrate surface, carbon remains on the interface between the oxide film and the substrate surface and forms a recombination center on the interface. Also, this develops charge capture and lowers the electric characteristic of a semiconductor such as the mobility of the thin-film transistor. Further, because the bonding state is not stabilized, an electric field continues to be applied, whereby the interface state is changed in time with a lost reliability.
Also, Japanese Patent Unexamined Publication No. Hei 4-177735 by the present applicant discloses that a bias is applied to a substrate using hydrogen of 100% to conduct plasma hydrogen cleaning on the semiconductor surface before forming a film by a sputtering unit. However, at the time of filing a Japanese Patent Application of the above Publication, because it has not been found that hydrogen radicals are effected on the single bond of carbon, a bias has been applied to the substrate to use the sputtering effect due to hydrogen ions, thereby cleaning the semiconductor surface.
For that reason, in order to make the interface characteristic excellent, because a balance of the effect obtained by removing the impurities and the damage caused by sputtering must be kept, the process margin cannot be increased so much. For that reason, processes which are capable of utilizing the plasma hydrogen cleaning have been limited.
Furthermore, not only the carbon contamination on the surface but also the carbon contamination in the film, using an organic silane source causes serious problems. A method of forming a film using normal tetraethyl silicate which has been well used is stated below. As the plasma CVD method, a substrate on which a film is formed is disposed within a chamber which has a parallel flat electrode and is capable of creating a vacuum in the chamber. In this situation, the one end of the parallel flat electrode is connected to the high-frequency source, connected to so-called cathode. The other one of the electrode is connected to the earth, and the substrate is disposed on the earth side electrode, that is, the anode side. Normal tetraethyl silicate is heated and increased in vapor pressure because it is liquid at room temperature, before being introduced into the chamber, or normal tetraethyl silicate bubbles with a carrier gas within a tank, and is then introduced together with the carrier gas into the chamber. Normal tetraethyl silicate resolved in plasma is characterized in that it forms a precursor and moves fluidly, thereby being capable of forming a film excellent in the step coverage property. The precursors that move on the substrate collide with each other, and also oxide ions, oxide radicals and ozones, and create an abstraction reaction to form SiOx. If a large amount of oxygen is introduced into the chamber, then the abstraction reaction from the precursor which is made of normal tetraethyl silicate on the surface facilitates, to form a film which is reduced in the amount of carbon but low in step coverage property.
When the amount of introduction of oxygen is slightly small, the step coverage property is improved, but the bond of carbon or oxygen and hydrogen exists much, thereby forming a film high in hygroscopicity. As a result of the measurement due to infrared absorption, the film is formed such that the absorption in the vicinity of 3660 cmxe2x88x922 is increased much as a time elapses. This exhibits that the absorption in the vicinity of 3660 cmxe2x88x922 is mainly caused by the Sixe2x80x94OH bond, and the formed film is hygroscopic.
As another method of forming a film using normal tetraethyl silicate, there is the atmospheric CVD method using ozone and heat. This is that normal tetraethyl silicate contained in a tank is bubbled with N2 on a substrate heated at 300 to 400xc2x0 C. and then heated in a reaction chamber, or oxygen is used for generating ozone through an ozonizer and then introducing it in the chamber. Since this method is high in step coverage property and also high in film forming rate, it is also used for an interlayer insulating film or the like, required for a multi-layer wiring such as LSI or DRAM memory. Thereafter, a so-called flatting operation is conducted in combination of etchback, SOG (SPIN ON GLASS), CMP (CHEMICAL MECHANICAL POLISHING), etc.
Among carbon contaminations on the substrate surface in a process of manufacturing the thin-film semiconductor device, particularly the impurities caused by a single bond (Cxe2x80x94C) of carbon which can be hardly removed by the conventional wet ashing or dry ashing are reduced, thereby reducing the deterioration of the electric characteristic caused by the impurities of carbon on the boundary where a variety of semiconductors are formed, the lowering of the reliability, and so on. In particular, the carbon contamination on the interface between the active layer semiconductor and the gate insulating film is reduced. Also, in the case of forming a film with an organic gas such as normal tetraethyl silicate as a source, the hygroscopicity and the content of carbon are increased with an improvement in step coverage property, resulting in a lack of the reliability and the no-good property of the semiconductor characteristic. Moreover, a large amount of oxygen is added to the organic silane gas such as normal tetraethyl silicate in order to reduce the content of carbon, to thereby lower the step coverage property, disconnect a wiring, etc., resulting in a lack of the reliability and the no-good property of the semiconductor characteristic.
The present invention has been made to eliminate the above-mentioned problems, and therefore an object of the present invention is to provide a method of manufacturing a semiconductor device which is capable of making the step coverage property excellent, reducing the content of carbon in comparison more than that in the conventional method, reducing the hygroscopicity and increasing the film forming rate. The method of the present invention enables the carbon impurities of the semiconductor boundary to be reduced, and carbon in the gate insulating film which is formed using an organic silane source to be reduced without losing the step coverage property.
The inventors have found that active hydrogen such as hydrogen radicals or hydrogen ions effectively operates to remove the impurities caused by the single bond of Cxe2x80x94C stuck on the substrate surface. This fact has been found under study for spreading the process margin of cleaning using the above-mentioned hydrogen sputtering effect. An excellent interface can be manufactured because the damage of the interface by sputtering is little, and active hydrogen effectively operates to remove the impurities of carbon. Also, in cleaning due to sputtering, because damages are caused by sputtering, the effect of removing the impurities need be balanced with the damages, thereby being incapable of increasing the process margin. However, because active hydrogen is used, no damage is caused by sputtering, and because the impurities can be removed, the process margin can be increased. Also, it has been found that, although sufficient effects can be obtained by only hydrogen radicals, with active oxygen such as oxygen radicals, ozone or oxygen ions being added thereto, its removing effect is increased. This is because hydrogen and oxygen radicals, etc. react with carbon bond, to form gas such as CHx, COx or CHO with the result that carbon is gasified.
In order to generate hydrogen radicals or hydrogen ions, for example, a substrate is disposed in a parallel-plate-electrode type plasma generating unit. In this situation, the substrate is preferably disposed at the side of an anode in order that the substrate is prevented from damages such as plasma ions, and if the substrate is allowed to be heated, elimination is effected by heat, thereby increasing that effect.
In the case of applying heat, if the substrate is made of a material relatively high in heat resistance such as quartz or Si wafer, since the substrate temperature can be set to 900xc2x0 C. or higher, the use of the chamber for the plasma generating unit which is made of quartz is effective. Also, in the case where metal low in melting point such as Al has already existed on a Si wafer or a quartz substrate, or in the case of using a glass substrate, etc., since the substrate temperature cannot be elevated so much, metal such as stainless steel may be conveniently used for the chamber for the plasma generating unit.
Upon applying a high-frequency power between the parallel plate electrodes with the introduction of hydrogen gas, plasma is generated. In plasma, neutral hydrogen radicals high in activity are generated together with hydrogen ions and electrons. The increase in high-frequency power is useful in increasing the amount of the active materials such as that radicals or ions. However, the use of microwave utilizing electron cyclotron resonance enables the amount of hydrogen radicals and ions to be further increased. The generated hydrogen radicals and ions reach the substrate surface, and then react with the single bond Cxe2x80x94C of carbon, thereby removing it. Carbon which has reacted and been gasified is exhausted by a pump.
Also, upon bringing heated hydrogen in contact with catalyst such as Rd/Al2O3, Pd/C or Ru/C, hydrogen radicals are generated by the catalytic action. Therefore, hydrogen radicals are carried up to the substrate surface without any damage of plasma, thereby being capable of removing the single bond of carbon.
In order to effectively remove Cxe2x95x90C, Cxe2x80x94O, Cxe2x95x90O, etc. except for the single bond of carbon, the use of oxygen radicals, or ozone or oxygen ions are very effective. Upon bringing oxygen radicals, etc., in contact with the bond of carbon, they are gasified in the form of COx, thereby enabling a so-called ashing process.
In order to generate active oxygen such as oxygen radicals, or ozone or oxygen ions, for example, a substrate is disposed in the plasma unit of the parallel plate electrode type. In this situation, in order to prevent the damages caused by ions in plasma, the substrate is preferably disposed at the side of an anode, and when the substrate is heated, elimination is effected by heat, to increase the effect.
Upon applying a high-frequency power between the parallel plate electrodes with the introduction of oxygen gas, plasma is generated. In plasma, neutral oxygen radicals or ozone high in activity are generated together with oxygen ions and electrons. In order to increase the amount of the radicals and so on, an increase in the high-frequency power is effective. However, the use of microwaves utilizing electron cyclotron resonance enables the amount of oxygen radicals and ions to be further increased. The generated oxygen radicals and ions reach the substrate surface, and then react with the carbon bond, thereby removing it. Carbon which has reacted and been gasified is exhausted by a pump.
Also, since a large amount of ozone is generated upon applying ultraviolet rays to oxygen gas, hydrogen radicals are carried up to the substrate surface without damages caused by plasma, thereby being capable of removing carbon single bond there.
In removing carbon on the substrate surface with active hydrogen and active oxygen, the use of both the active hydrogen and oxygen is very effective. First, carbon impurities consisting of carbon bonds except for the single bond of carbon are removed using active oxygen, and thereafter the carbon impurities mainly consisting of the single bonds of carbon can be removed by active hydrogen. Alternatively, active hydrogen and active oxygen are so mixed as to remove those carbon impurities simultaneously.
In order to generate active hydrogen and active oxygen, hydrogen gas and oxygen gas are introduced simultaneously into a parallel plate type plasma generating unit or a microwave plasma generating unit using an electron cyclotron, which are capable of generating plasma, etc., thereby generating plasma to generate hydrogen ions, hydrogen radicals, oxygen ions, oxygen radicals and ozone simultaneously so that carbon on the substrate surface is removed, and removed carbon is exhausted by a vacuum pump.
In particular, when hydrogen and oxygen are introduced into a processing chamber to which ultraviolet rays are applied with catalyst such as Rd/Al2O3, Pd/C or Ru/C, hydrogen generates hydrogen radicals due to a catalytic reaction, and oxygen generates ozone due to ultraviolet rays, thereby being capable of removing the carbon impurities on the substrate surface without the substrate being damaged by plasma.
Instead of using hydrogen and oxygen to generate active hydrogen and active oxide, H2O may be used. H2O is introduced in the parallel plate type plasma generating unit or the microwave plasma generating unit using the electron cyclotron, which are capable of generating plasma.
There are some methods of introducing H2O. There is a method in which H2O in a tank with an inactive gas such as He, Ne or Ar as a carrier gas is bubbled, to thereby carry H2O into a processing chamber as gas. Also, there is a method in which all portions of from the tank containing H2O to the processing chamber are heated so the vapor pressure of H2O is elevated, and gas is carried as it is.
Introduced H2O is decomposed by plasma, to thereby generate hydrogen ions, hydrogen radicals, oxygen ions, oxygen radicals and ozone simultaneously. As a result, the carbon impurities on the substrate surface can be removed. The removed carbon is exhausted by a vacuum pump.
FIG. 1 is a graph representative of the degree of removing the carbon impurities on the substrate surface through XPS, which was obtained by the present invention. There have been measured through XPS a substrate surface 11 (indicated by a broken line in FIG. 1) which has been subjected to photoresist coating, pre-baking, light-exposure, developer, post-baking, resist separation, and thereafter left within a clean room for one day, a substrate surface 12 (indicated by a dashed line in FIG. 1) which has been subjected to dry aching and wet aching, and a substrate surface 13 (indicated by a solid line in FIG. 1) from which the carbon impurities have been removed in accordance with the present invention after the substrate surface 12 was subjected to photoresist coating, pre-baking, light-exposure, developer, post-baking, resist separation, and thereafter left within a clean room for one day. As a measurement condition, the angle of a detector is set to 15xc2x0 in order to obtain the information as to the surface as much as possible, an area of 1 mm"PHgr" on the substrate surface has been measured. The horizontal axis represents a bonding energy with a unit of eV whereas the vertical axis represents the intensity of the detector with an arbitrary unit.
It is found from the graph of FIG. 1 that a peak in the vicinity of 284.8 eV is increased at the graph 11 before and the graph 12 after conducting the dry aching and wet aching, and all of other peaks are decreased. Also, it is found that a peak in the vicinity of 284.8 eV is also remarkably decreased in a graph 13 using the present invention.
It is presumed that the reason why the peak does not become completely zero even using the present invention is that measurement is not conducted just after the carbon impurity removing process of the present invention, but because a time interval is inserted before the measurement after the carbon impurities have been removed in accordance with the present invention, there exist the carbon impurities stuck thereto. However, the carbon impurity removing process of the present invention is effective as compared with the case without such a carbon impurity removing process. Using the present invention, the carbon contamination having the single bond of carbon can be reduced.
Moreover, in the case where an oxide film is formed through the plasma CVD technique using normal tetraethyl silicate, oxygen and normal tetraethyl silicate have been mixed to form a film as a method of reducing carbon in the film. However, it is found out that using active hydrogen such as hydrogen radicals and hydrogen ions in the film has its effect. Active hydrogen such as hydrogen radicals and hydrogen ions react with carbon to form CHx and gasify carbon. In particular, the bond of Cxe2x80x94C which is the single bond of carbon is decoupled into CH4 and Cxe2x80x94OH. Thus, carbon is gasified, thereby being capable of removing carbon in the film.
Because hydrogen has the effect of a so-called decarbonization for carbon in comparison with oxygen, and atoms are small, the sputtering effect of hydrogen ions on the film and the substrate is to the degree where it can be almost ignored in comparison with oxygen. For that reason, in the case where normal tetraethyl silicate, oxygen and hydrogen are mixed together to form a film through the plasma CVD method, the ratio of normal tetraethyl silicate to oxygen is determined to set a film forming rate so that the step coverage property and the productivity are excellent, and a system in which hydrogen is mixed with other components for decarbonization is taken. In particular, the effect is large when hydrogen 0.01 to 0.5 times as much as normal tetraethyl silicate is introduced.
As a result, while the precursor from normal tetraethyl silicate generated by plasma, oxygen ions, ozone and oxygen radicals repeat the surface reaction for forming a film on the substrate surface, the precursor fluidrizes on the substrate surface while it changes into various kinds of precursors, to thereby form an oxide film excellent in the step coverage property. In this case, during a process in which the oxide film is formed with a reaction of the precursor with oxygen ions, ozone or oxygen radicals, hydrogen ions and hydrogen radicals react with the carbon atoms on the substrate surface to gasify carbon. Gasified carbon is exhausted by a vacuum pump.
In the case where the present invention is applied to the formation of a film using atmospheric CVD, there is used a catalytic method for forming a part of hydrogen into hydrogen radicals. As the catalyst, 3d-transition metal such as platinum, paradium, reduced nickel, cobalt, titanium, vanadium, or tantalum, aluminum, nickel, metal compound such as platinum-silicon, platinum-chlorine, platinum-rhenium, nickel-molybdenum, cobalt-molybdenum, the mixture or compound of the above-mentioned transition metal and alumina, silica gel or the like, or Raney cobalt, ruthenium, paradium, nickel or the like, or the mixture or compound of those materials and carbon are proper. They are used in the grain-like, net-like, cotton-like or particle-like state.
It should be noted that material that remarkably increases the initial absorption rate of the reactive material at a low melting point, and material that contains alkali metal such as sodium which is liable to be gasified within the material, for example, copper, tungsten, or the like is not preferable. According to the experiment, it has been found that the catalyst is remarkably deteriorated at a temperature equal to or higher than the decomposition temperature of the reaction material. The amount of catalyst and the density thereof are concerned with the effective contact area with the reactive gas and may be adjusted as occasions demand. Hydrogen is allowed to pass through heated catalyst to generate active hydrogen radical. Oxygen is allowed to pass through ozonizer to generate active ozone.
In the plasma CVD method, hydrogen radicals are generated by plasma, and in the atmospheric CVD method, hydrogen radicals are generated by the catalysis method. They may be reversed. Active hydrogen radicals have been generated through the catalysis method in advance, and then it may be introduced into the plasma CVD unit. Alternatively, active hydrogen radicals have been formed by electric discharge in advance, and thereafter they may be mixed together by a gas nozzle of the atmospheric CVD unit.
Furthermore, in the case of forming an oxide film using normal tetraethyl silicate, oxygen is used as a source for using active oxygen such as oxygen radicals, oxygen ions and ozone. However, in the present invention, H2O can be used for using hydrogen radicals or hydrogen ions in addition thereto. It should be noted that since H2O and normal tetraethyl silicate are high in reactivity, when they are mixed together within the pipe before they react with each other on the substrate, the pipe may be blocked. In the plasma CVD method, a lead-in pipe for normal tetraethyl silicate and a lead-in pipe for H2O may be preferably separated from each other.
Because the use of organic silane including F such as FSi(OC2H5)4 instead of normal tetraethyl silicate enables SiOx of F dope small in the content of carbon and lower in dielectric constant than SiOx to be manufactured, the capacity of the interlayer insulating film between the wires in the lateral direction can be reduced. Also, in the case of using organic silane system containing carbon as a source, the present invention is very effective in improving decarbonization and the step coverage property as well as in ensuring the film forming rate.
According to the present invention, in particular, in the top-gate type thin-film transistor (TFT), after the formation of a semiconductor layer which is an active layer, the active layer is patterned, and thereafter a gate insulating film is formed. For that reason, the carbon contamination on the surface of the active layer is strong, and therefore the formation of the gate insulating film without removing it causes the reliability to be lowered with the no-good property of the transistor characteristic. Also, in the manufacture of a thin-film transistor where an oxide film having an organic silane source is formed on the gate insulating film, it is very important to obtain an oxide film which is excellent in the step coverage property, reduced in carbon and low in hygroscopicity.
In the manufacture of the semiconductor device in accordance with the present invention, the carbon impurities which could not be removed by the conventional dry ashing or wet ashing can be remarkably reduced, thereby being capable of remarkably reducing the carbon impurities in the oxide film using an organic silane source in the film. In particular, the present invention is effective in removing the impurities or the contamination including the single bond of carbon represented by Cxe2x80x94C, thereby cleaning the interface of the semiconductor at the time of forming laminate layers of the semiconductor. Also, the present invention reduces the carbon impurities in the oxide film using the organic silane source, thereby obtaining the remarkably effects such as an improvement in the electric characteristic of the thin-film semiconductor device, an improvement in the reliability, and so on.